Biomedical Engineering Reference
used as a building block in plastics manufacture, particularly production of methyl
methacrylate, which is polymerized to form PMMA, also known as plexiglass. Butanol, too,
is used as a solvent. It is also used to produce butyl esters, some of which are flavorings used
in the food industry, and as a precursor for plastics. Recently, butanol has been proposed for
use as a transportation fuel. Ethanol is found in numerous beverages and is used as a
ABE fermentations are characterized by the presence of two distinct phases. The first
phase is called acidogenesis. During this phase acetate and butyrate are formed and cell
growth is observed. This phase is also characterized by a media pH greater than 5. As the
pH decreases to below 5 with the production of more acid, cell growth ceases and the cells
enter the second phase, known as solventogenesis. During solventogenesis acetone, butanol
and ethanol are produced. Often during solventogenesis acids produced during acidogenesis
are taken up by cells and reduced to form alcohols. This action increases extracellular pH
and reduces the pH gradient across the cell membrane.
Glycerol is produced primarily from petrochemical catalysis, although some is produced
as a by-product of soap manufacturing. An increasing supply of glycerol is also produced
by the biodiesel industry as a result of the hydrolysis of fatty acids from glycerol prior to
methyl esterification of the fatty acids to produce biodiesel. Glycerol is an excellent
substrate for the production of succinate and propionate, as described below. It can be
used also to produce ethanol using recombinant Escherichia coli (Dharmadi et al ., 2006 ).
Glycerol is a valuable chemical in its own right for food and pharmaceutical production,
as well as the production of many other consumer products. One example is the production
of propylene glycol, used in antifreeze, by the catalytic upgrading of glycerol (Dasari
et al ., 2005 ).
Production of glycerol by fermentation has not been pursued commercially since before
the Second World War. During the First World War, Germany produced glycerol with yeast
through a process called sulfite steering (Wang et al ., 2001). In this process sulfite ions were
added to the media to form complexes with acetaldehyde. These complexes cannot be
reduced by NADH to ethanol, so the yeast cell must reduce dihydroxyacetone phosphate
(DHAP) to glycerol in order to maintain its redox balance. This process results in an
approximately 25% mass yield of glycerol from glucose. Due to low yield and the low cost
of petroleum, this process was abandoned.
Another process that also was used for microbial production of glycerol was the
neutral pH process (Wang et al ., 2001). This process involved fermenting glucose with
yeast at pH 7. At neutral pH acetaldehyde dehydrogenase activity is increased; thus,
acetaldehyde is oxidized to acetate and NADH is formed (Wang et al ., 2001 ). The cell
then reduces DHAP to glycerol to maintain its redox balance. This process also suffered
from low yields and the production of other side products, such as acetate and ethanol,
and was abandoned. (Wang et al ., 2001 ).
The microbial glycerol production process that has generated the highest yields to date is
the aerobic conversion of glucose by osmotolerant yeast (Wang et al ., 2001 ; Overkamp
et al ., 2002). Glycerol is a protectant against high osmotic pressures; thus, it is produced by
osmotolerant yeast under osmotic stress (Blomberg and Adler, 1992). Mass yields of
glycerol from glucose of up to 64% have been reported using osmotolerant yeast (Zhuge
et al ., 2001 ).